Ultrasonic sensors which are used for distance measurement between vehicles and obstructions are already known. The sensors have an oscillating diaphragm, which is generally excited to oscillate in resonance via a piezoelement. The sound signal produced in this way is emitted by the diaphragm of the ultrasonic sensor, reflected by an obstruction, and received again by the same or a neighboring ultrasonic sensor. The distance of the sensor to the obstruction is able to be determined from the runtime. In general, the piezoelement is positioned on the floor of a diaphragm cup, which is mounted in the bumper of the vehicle, for example, the piezoelement being connected to analysis electronics in such a way that the entire device forms the corresponding ultrasonic sensor. In order that the sound produced by the piezoelement may be emitted and/or also received appropriately, the diaphragm cups are shaped in such a way that they have a natural resonance in the range of the ultrasound waves used. The resonance is determined in this case by the dimensions and the oscillation characteristics of the diaphragm cup, in particular by the layer thickness of the diaphragm.
Preferably, a metal, in particular aluminum, or a ceramic material is used as the material for the diaphragm cups. So that the sensors are not unnecessarily noticeable in the vehicle contour and that the sensors may be protected from environmental influences, it is necessary to provide the sensors with appropriate coatings and/or lacquerings. However, depending on the material, these layers may not be applied directly to the diaphragm cup. Multiple materials either do not adhere sufficiently or influence the sound propagation of the ultrasonic waves negatively. Depending on the layer thickness, the application may also result in the resonance frequency of the diaphragm cup as a whole being changed, so that the diaphragm may no longer be excited in resonance to oscillate.